The use of optical sources of radiation such as lasers, light emitting diodes (LED's), arc lamps, and other sources of incoherent radiation to perform chemical analysis and identification has been known for many years and such sources have been used in a wide range of chemical analysis and analytical instruments. Among these instruments, capillary electrophoresis, high performance liquid chromatography, flow cytometry, Raman and fluorescence microscopy and spectroscopy are emerging as powerful analytical tools for a wide range of biological and chemical research, as well as clinical, industrial, and governmental applications. These instrumental techniques are being increasingly used in commercial and governmental applications such as product inspection during the manufacture of pharmaceutical and medical products, manufactured food and chemical products, environmental testing, and other applications.
When a sample is exposed to radiation (e.g. infrared (IR) radiation, visible light, or ultraviolet (UV) radiation) at a given frequency, some of the radiation is transmitted through the sample. Some of the radiation is elastically scattered and retains the same frequency as the incident radiation. Some of the radiation is absorbed in the sample. The absorbed radiation is either re-emitted after interaction with the sample or converted to thermal energy in the sample. The re-emitted radiation is sometimes referred to as inelastically scattered radiation. The inelastically scattered radiation is re-emitted as fluorescence or phosphorescence at wavelengths longer than, or frequencies shorter than the irradiation frequency, and a small fraction is re-emitted as Raman scattered radiation. Fluorescence or phosphorescence emissions are red shifted from the excitation frequency and have a spectral distribution that is relatively independent of the excitation frequency. Raman emissions are dependent on excitation frequency and are measured as a sum or difference frequency from the excitation frequency. Absorption of radiation requires that the energy of the exciting photon is higher than the first excited state of the molecule being excited. Raman emissions can be either blue (anti-Stokes) or red (Stokes) shifted from the excitation frequency by an amount determined by the rotational and vibrational bonds within the molecules being irradiated. Raman scattering efficiency is typically very low compared to fluorescence. However, when the energy of the excitation radiation corresponds to strong absorption bands of the analyte, a resonance effect can amplify the Raman signal by many orders of magnitude.
Fluorescence, phosphorescence and Raman techniques are employed for chemical detection and identification in a wide range of instruments. Lasers, LED's and other sources are typically used for excitation. Chemical separation technologies such as capillary electrophoresis (CE), high performance liquid chromatography (HPLC), capillary electrochromatography (CEC), and various related instrumental forms allow rapid separation of complex chemical and biochemical mixtures into component elements. Laser induced fluorescence (LIF) allows the sensitive detection of separated elements or analytes where limits of detection have been demonstrated into the zeptomole range.
A major drawback to the use of fluorescence detection is that the vast majority of chemicals do not absorb strongly at visible or infrared wavelengths where simple, inexpensive lasers emit. In order to match the emission wavelength of these desirable lasers and other sources, fluorescence detection performed above about 300 nm requires derivatization of most analytes with a fluorescent dye tag prior to analysis. This is highly inconvenient, especially at low analyte concentrations, and for compounds that lack appropriate functional groups. For multifunctional compounds such as proteins, it is very difficult to ensure stoichiometrical reactions that can result in complex mixtures after derivatization. Derivatization limits the types of molecules that can be studied, can lower overall detection sensitivity, reduces the ability to find unexpected analytes in complex mixtures, and may alter the very chemistry being studied. Derivatization is commonly employed in analytical instruments today because of the lack of suitable lasers in the deep UV with small size, reasonable power consumption, and acceptable cost. The sensitivity and specificity as well as simplicity and ease of use of analytical instruments have been demonstrated to be considerably enhanced when combined with a laser that emits in the deep UV between 200 nm and 300 nm. These advantages were demonstrated using lasers that are unacceptable in a commercial instrument application because of their size, weight, power consumption, and/or cost. The advantages of deep UV excitation are true for detection in chemical separation instruments as well as in point-by-point or global imaging instruments, and other instruments using optical methods of detection. In addition to fluorescence, Raman spectroscopy is a powerful analytical method for determining properties of unknown materials. Narrow Raman emission bands carry a great deal more information on molecular structure, in contrast to broadband fluorescence emission. It has been hampered as an analytical method by the fact that normal Raman scatter cross-sections of materials are typically very small. Another problem that has hampered Raman spectroscopy as an analytical method is fluorescence background from many materials at wavelengths of interest in obtaining Raman spectra. In the visible portion of the spectrum where many types of lasers are available, many materials emit fluorescence that overwhelms the small Raman emissions from a sample. To alleviate this problem instruments have been developed which operate in the near infrared where fluorescence background is greatly diminished or, in some cases, eliminated. The problem remains that Raman scatter cross-sections in the infrared are small and powerful lasers are needed to obtain Raman spectral data. In addition to their other problems, these powerful lasers sometimes cause sample damage.
Several advantages arise for Raman spectroscopy and analysis when using a deep ultraviolet laser to irradiate a sample. First, scatter cross-sections are inversely proportional to the fourth power of excitation wavelength. Thus, as the excitation wavelength of a laser is moved from the near infrared to the ultraviolet, an increase over 100 times in Raman scattering typically occurs. Second, when excitation occurs below about 250 nm, fluorescence background is eliminated within the Raman spectral range of most samples. This ubiquitous fluorescence, which is a major impediment for visible wavelength Raman studies, does not occur for UV spectral studies below about 270 nm. This is because at these high energies the excited state of most molecules in a condensed phase relaxes by means of fast radiationless processes before it has time to fluoresce. Third, when excitation occurs within an electronic absorption band of the sample, a resonance effect causes dramatic increases in Raman signal strength, often over one million to one hundred million times, thus eliminating the need for powerful lasers. Many types of materials have strong absorption bands in the deep ultraviolet below about 300 nm. These include organic and biological materials as well as a large range of other materials. And fourth, resonance Raman bands are enhanced preferentially for those molecular bonds associated with the electronic absorption, thus considerably simplifying the Raman spectra and making them more easily interpreted. For these types of samples, deep ultraviolet Raman spectroscopy and single band Raman imaging can be important analytical methods.
The state of the art of Raman spectroscopy can still be advanced by the application of additional deep UV laser sources to such analytic instruments (particularly low power and/or small size sources), by using optical components that allow further reductions in system size, weight, power consumption, and the like.
Various analytic instruments and analytic methods have been described previously. Patents having such teachings include:
1. U.S. Pat. No. 6,287,869, entitled “Analytic Instrument Using a Sputtering Metal Ion Laser” by Hug, et al.;
2. U.S. Pat. No. 6,002,476, entitled “Chemical Imaging System” by Treado;
3. U.S. Pat. No. 5,623,342, entitled “Raman Microscope” by Batchelder, et. al.;
4. U.S. Pat. No. 5,442,438, entitled “Spectroscopic Apparatus and Methods”, by Batchelder, et. al.; and
5 U.S. Pat. No. 5,194,912, entitled “Raman Analysis Apparatus” by Batchelder, et. al.
The teachings of each of these patents are hereby incorporated herein by reference as if set forth in full. With the exception of the '869 patent, a feature that universally distinguishes these patents from some embodiments of the invention is that they are related to apparatus utilizing visible or infrared wavelengths. The teachings of the '869 patent will be discussed further herein after.
Additional publications providing teachings about analytic instruments and methods include:    1. Ianoul, A., T. Coleman, and S. A. Asher, “UV Resonance Raman Spectroscopic Detection of Nitrate and Nitrite in Wastewater Treatment Processes”, Anal. Chem., Vol. 74, pp. 1458-1461, 2002.    2. Storrie-Lombardi, M. C., W. F. Hug, G. D. McDonald, A. I. Tsapin, and K. H. Nealson. “Hollow cathode ion laser for deep ultraviolet Raman spectroscopy and fluorescence imaging”. Rev. Sci. Instruments, 12, 4452-4459, December 2001    3. Sparrow, M. C., J. F. Jackovitz, C. H. Munro, W. F. Hug, and S. A. Asher, “A New 224 nm Hollow Cathode UV Laser Raman Spectrometer”, J. App. Spectroscopy, Vol. 55, No. 1, January 2001.    4. Gillespie, S. R. and J. W. Carnahan, “Ultraviolet Quartz Acousto-optic Tunable Filter Wavelength Selection for Inductively Coupled Plasma Atomic Emission Spectrometry”, J. App. Spectroscopy, Vol. 55, No. 6, 2001.    5. Wu, Q, T. Hamilton, W. H. Nelson, S. Elliott, J. F. Sperry, and M. Wu, “UV Raman Spectral Intensities of E. Coli and Other Bacteria Excited at 228.9, 244.0 and 248.2 nm”, Anal. Chem. Vol. 73, No. 14, pp. 3432-3440, Jul. 15, 2001.    6. McCreery, R. L., “Raman Spectrocopy for Chemical Analysis”, John Wiley & Sons, ISBN #0-471-25287-5, 2000.    7. Munro, C. H., V. Pajcini, and S. A. Asher, “Dielectric Stack Filters for Ex Situ and In Situ UV Optical-Fiber Probe Raman Spectroscopic Measurements”, App. Spect., Vol. 51, No. 11, pp 1722-1729, 1997.    8. Morris, H., C. et. al., “Liquid Crystal Tunable Filter Raman Chemical Imaging”, App. Spect., Vol. 50, No. 6, pp. 805-811, 1996.    9. Turrell, G, et. al., “Raman Microscopy”, Academic Press Ltd., ISBN#0-12-189690-0, 1996.    10. Macleod, A., “Thin-Film Optical Filters”, McGraw-Hill, ISBN#0-07-044694-6, reprinted 1989    11. Treado, P. J., and M. D. Morris, “A Thousand Points of Light: The Hadamard Transform in Chemical Analysis and Instrumentation”, Anal. Chem., Vol 61, No. 11, pp. 722-734, Jun. 1, 1989.    12. Asher, S. A., “Raman Spectroscopy of a Coal Liquid Shows That Fluorescence Interference Is Minimized with Ultraviolet Excitation”, Science, Vol. 225, 20 Jul. 1984.    13. Wolf, W. L. ed., Handbook of Military Infrared Technology, Office of Naval Research, Dept. of the Navy, Washington, D.C., pp. 286-306, 1965.    14. Military Standardization Handbook, MIL-HDBK-141, Section 20, 5 Oct. 1962. (angle dependence, p. 20-11)    15. Jenkins, F. A. and H. E. White, Fundamentals of Optics, (McGraw Hill), 1957.    16. Mooney, C. F., and A. F. Turner, “Infrared Transmitting Interference Filters, Proceedings of the Conference on Infrared Optical Materials, Filters, and Films”, Engineering Research and Development Laboratories, Fort Belvoir, Va. (1955).
Each of the publications is hereby incorporated herein by reference as if set forth in full. Additional references having such teachings are found in the background of previously incorporated U.S. Pat. No. 6,287,869. These additional references are hereby incorporated therein by reference as if set forth in full.
The teachings in U.S. Pat. No. 6,287,869 provided improved and more commercially viable analytical systems by providing deep UV radiation sources in the form of sputtering metal ion hollow cathode lasers for use in such systems. These lasers provided improved choice of emission wavelengths, improved duty cycle, reduced size, reduced power consumption, reduced complexity, reduced cost and improved reliability. A need remains, however, for other UV radiation sources that provide further size reductions, weight reductions and cost savings for use in analytical instruments.
Semiconductor optical sources including lasers and light emitting diodes may be the ultimate forms of radiation sources for analytical instruments since they typically are very small, have low power consumption and can be produced at low cost. Research on blue and ultraviolet lasers has been ongoing for many years with significant progress. But several major technical roadblocks have impeded progress in demonstrating sources which operate in the deep UV, especially at wavelengths less than about 300 nm. Wide bandgap semiconductor materials based on alloys of aluminum and gallium nitrides plus various dopants are being extensively investigated to fulfill the need for UV semiconductor sources. Pure aluminum nitride (AlN) has a bandgap of 6.2 electron volts (eV), which corresponds to an emission wavelength of 200 nm. Pure gallium nitride (GaN) has a bandgap of 3.4 electron volts, which corresponds to an emission wavelength of 365 nm. By alloying these materials any emission wavelength between 200 nm and 365 nm can theoretically be produced. To produce a semiconductor laser with emission wavelength less than 250 nm requires an aluminum mole fraction in the alloy greater than about 60%. The major roadblocks to producing deep UV semiconductor sources have been:                1. the inability to adequately p-dope AlGaN materials with aluminum content greater than a few percent (i.e. measure as mole fraction);        2. the inability to make low resistance, ohmic, contacts to AlGaN alloys with aluminum content greater than a few percent;        3. Lattice mismatch with available substrate materials resulting in high defect density in active region;        4. Inability to form facets with low losses at deep UV wavelengths; and        5. Difficulty in forming waveguiding layers in high Al-content AlGaN alloys.A need exists in the art for overcoming these roadblocks. The first two items above are the most significant of these roadblocks.        
The general idea of using electron beams to pump various materials to produce laser output is not new. In fact, a significant amount of literature and some patents exist on various devices and applications based on these methods. These publications date back to 1964. These publications include the following articles and patents, each of which is incorporated herein by reference:    1. C. E. Hurwitz and R. J. Keyes, “Electron-beam-pumped GaAs Laser”, App. Phys. Lett, Vol. 5, No. 7, pp. 139-141, (Oct. 1, 1964).    2. C. E. Hurwitz, “Efficient ultraviolet laser emission in e-beam excited ZnS”, Appl. Phys. Lett., v. 9, N. 3, pp 116-118 (1966)    3. C. E. Hurwitz, High Power and Efficiency in CdS Electron Beam Pumped Lasers, Applied Physics Letters, vol. 9, No. 12, Dec. 15, 1966, pp. 420-423.    4. C. E. Hurwitz, “Electron-beam pumped lasers of CdSe and CdS”, App. Phys. Lett., Vol. 8, No. 5, pp. 121-124, (March 1966)    5. C. E. Hurwitz, “Efficient visible lasers of CdSeSe by electron-beam-excitation”, Applied Physics Letters 8, 243 (1966)].    6. C. A. Klein, “Further remarks on electron beam pumping of laser materials”, Appl. Optics, 5, 12, 1922 (1966)    7. Nasibov et al., “Electron-Beam Tube with a Laser Screen”, Sov. J. Quant. Electron., vol. 4, No. 3, September 1974, pp. 296-300.    8. O. V. Bogdankevich et al., Application of Electron Beam Pumped Semiconductor Lasers to Projection Television, IEEE Journal of Quantum Electronics, vol. 13, No. 9 (Sop. 1977), p. 66D.    9. O. V. Bogdankevich, The Use of Electron-Beam Pumped Semiconductor Lasers in Projection Television, IEEE Journal of Quantum Electronics, vol. OF-14, No. 2, February 1978, pp. 133-135.    10. Bogdankevich et al., “Multilayer GaAs—AlAs Heterostructure Laser Pumped Transversely by an Electron Beam”, Sov. J. Quantum Electron. 10 (6), June 1980, pp. 693-695.    11. Bogdankevich et al., “Influence of Doping of Ga.sub.0.68 Al.sub.0.32 As on its Cathodoluminescence and Threshold Current Density of a Laser Pumped by an Electron Beam”, Sov. J. Quant. Electron., 11 (1), January 1981, pp. 119-121.    12. B. Kozlovskii, A. Nasibov, et. al., Sov. J. Quant. Electr., 12, 505 (1982)    13. E. Markov, V. Smirnov, V. Khryapov, “Physics and technical application AB Semiconductors”, V Conf., Prod., Vilnius, SU, 3, 131 (1983)    14. O. V. Bogdankevich et al., Distribution of the excitation density in electron-beam-pumped semiconductor lasers, Sov. J. Quantum Electron. 13 (11), November 1983, pp. 1453-1459.    15. Tong, F., R. M. Osgood, A. Sanchez, and V. Daneu, “Electron-beam-pumped two-dimensional laser array with tilted mirror resonator”, App. Phys. Letters (ISSN00003-6951), Vol. 52, pp. 1303-1305, Apr. 18, 1988    16. I. Akimova, V. Kozlovskii, et. al, “The influence of stoichiometry in A2B6 monocrystal compounds on the characteristics of a semiconductor electron-beam pumped laser”, Proc. Of Lebedev Phys. Inst., Nova Science Publ., USA, v. 177, pp. 195-233, 1988.    17. A. Nasibov, V. Kozlovsky, Y. Skasyrsky, “Deep blue and ultraviolet e-beam pumped semiconductor lasers”, SPIE Vol. 1041, Metal Vapor Deep Blue and Ultraviolet Lasers, Los Angeles, Calif., 17-20 Jan. 1989.    18. Molva, E., R. Accomo, G. Labrunie, J. Cibert, C. Bodin, Le Si Dang, and G. Feuillet, “Microgun-pumped semiconductor laser”, App. Phys. Letters, Vol. 62(8), pp. 796-798, Feb. 22, 1993    19. O. V. Bogdankevich, “Electron-beam-pumped semiconductor lasers”, Quantum Electronics, Vol 24, No. 12, pp. 1031-1053, 1994.    20. D. Herve, R. Accomo, E. Molva, L. Vanzetti, J. J. Paggel, L. Sorba, and A. Francoisi, “Microgun-pumped blue lasers”, App. Phys. Letters, Vol. 67(15), pp. 2144-2146, Oct. 9, 1995    21. V. I. Kozlovshy, A. B. Krysa, Y. K. Skyasyrsky, Y. M. Popov, w. S. DenBaars, “Electron beam pumped MQW InGaN/GaN laser”, MRS Internet J. Nitride Semicond. Res. 2, 38 (1997).    22. J. M. Bonard, J. D. Ganiere, L. Vanzetti, et. al., “Transmission electron microscopy and cathodoluminescence studies of extended defects in electron-beam-pumped Zn1-xCdxSe/ZnSe blue-green lasers”, J. App. Phys., (83) 4 p 1945 (15 Feb. 1998)    23. S. Krivoshlykov, “Compact high-efficiency electron-beam-pumped semiconductor laser operating at room temperature”, BMDO Phase I SBIR 1999    24. Nicholls, J. E., B. Lunn, et. al., “Electron-beam-pumped near-UV semiconductor laser emission”, EPSRC reference number GR/L27206, University of Hull, UK.    25. J. R. Packard, et. al., “Electron Beam Laser”, U.S. Pat. No. 3,757,250, Sep. 4, 1973    26. R. R. Rice, et. al., “Vertical cavity electron beam pumped semiconductor lasers and methods”, U.S. Pat. No. 5,807,764, Sep. 15, 1998    27. R. R. Rice, et. al., “Vertical cavity electron beam pumped semiconductor lasers and methods”, U.S. Pat. No. 5,677,923, Oct. 14, 1997.    28. D. A. Campbell, et. al., “Zinc Oxide Laser”, U.S. Pat. No. 3,505,613, Apr. 7, 1970.
A need exist in the field for extending the use of electron beam pumping to additional materials and applications so as to enable improved radiation sources (e.g. lasers and incoherent sources) which in turn may enable improved applications for such sources, e.g. improved chemical analysis methods and devices.